A screw is a type of threaded fastener, a device used to hold objects together. It consists of a shaft, which may be cylindrical or conical, and a head. The shaft has a helical ridge formed on it that assists in fixing the screw and stops it from falling out. The head is used to grip the screw when driving it in, and stops the screw from passing right through the material being fastened.

Screws are more versatile than nails: they can be taken out and replaced. Nails usually hold only by friction.

A screw that is tightened by turning it clockwise is said to have a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread.

A bolt is a screw that passes right through the workpiece and is fastened by a nut on the other side. This is a very common way of holding together temporary and permanent constructions. It avoids the need for making a threaded hole to fit the screw. An unthreaded hole is known as a clear hole.

A small bolt is called a machine screw.

The thread on a bolt often occupies only part of the shaft, the remainder of the shaft being clear. A bolt whose shaft is threaded along its whole length is called a set screw.

A screw is a specialized application of the wedge or inclined plane. It contains a wedge, wound around an interior cylinder or shaft, that either fits into a corresponding plane in a nut, or forms a corresponding plane in the wood or metal as it is inserted. The technical analysis (see also statics, dynamics) to determine the pitch, thread shape or cross section, coefficient of friction (static and dynamic), and holding power of the screw is very similar to that performed to predict wedge behavior. Wedges are discussed in the article on simple machines.

Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forces. When, for example, two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough that the friction between the two bars can overcome the longitudinal force. If the bars slip then the bolt may be sheared in half, or friction between the bars (called fretting) may weaken them. For this type of application, high-tensile steel bolts are used and these should be tightened with a torque wrench.

High-tensile bolts are usually in the form of hexagonal cap screws with an ISO strength rating stamped on the head. The strength ratings most often used are 8.8 and 12.9. The number before the point is the ultimate tensile strength in N/mm2 (or MPa) divided by 100. This is the stress at which the bolt will fail, i.e. break in half.

The number after the point is the yield strength as a percentage of the ultimate tensile strength, divided by 10. Yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2%.

Mild steel bolts have a 4.6 rating. High-tensile bolts have an 8.8 rating or above.

A variety of tools exist to drive screws into the material to be fixed. The hand-tool used to drive slot-headed and cross-headed screws is called a screwdriver. A power tool that does the same job is a power screwdriver. The hand-tool for driving cap screws and other types is called a spanner (UK usage) or wrench (US usage).

Slot head is driven by a flat-bladed screwdriver.

Cross-head, or Phillips screw has an X-shaped slot and is driven by a cross-head screwdriver, designed originally for use with mechanical screwing machines, intentionally made so the driver will ride out, or cam out, under strain to prevent over-tightening. The Phillips Screw Company was founded in Oregon in 1933 by Henry Phillips, who bought the design from J. P. Thompson. Phillips was unable to manufacture the design, so he passed the patent to the American Screw Company, who were the first to manufacture it.

Pozidriv is patented, similar to cross-head but designed not to slip, or cam-out. It was jointly patented by the Phillips Screw Company and American Screw Company.

Hexagonal or hex screw head has a hexagonal hole and is driven by a hexagonal wrench, sometimes called an Allen key, or a power tool with a hexagonal bit.

Robertson drive head has a square hole and is driven by a special power-tool bit or screwdriver (this is a low-cost version of the hex head for domestic use).

Torx head has a splined socket and receives a driver with a splined shaft.

Tamper-proof torx is similar to torx but the drive socket has a projection to prevent a standard torx driver being inserted.

Tri-Wing screws are used by Nintendo on its Gameboys. This discourages even minor home repairs to the units.

The diameter of a metric screw is usually specified in millimetres (mm) prefixed by the capital letter M, as in "M5" for a 5mm diameter screw. The diameter of a screw is the outer diameter of the thread, which is approximately equal to the diameter of the shaft before a thread was cut in it.

Metric screw threads are available in coarse and fine versions. The coarse thread is by far the more common. Fine metric threads are sometimes found in electronic equipment made in the Far East.

Before the metric system was common, many engineering companies had their own standard screw sizes. The first person to create a standard (in about 1841) was the English engineer Sir Joseph Whitworth. Whitworth screw sizes are still used, but mainly for repairing old machinery. This system had two thread sizes: coarse (BSW) and fine (BSF). The thread angle was 55°.

A later non-metric standard in the UK was the BA system, named after the British Association for Advancement of Science. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used.

The USA has its own system, usally called SAE, for Society of Automotive Engineers[?]. These screws are sometimes found outside the USA in personal computers based on the IBM PC specification. There must be millions of PCs outside the USA that have metric screws jammed into non-metric holes!

A screw is slang for a prison officer[?]. Named so because, during the Victorian period, prison inmates were given pointless tasks to perform such as the crank, where an inmate would have to crank a handle round and round, a certain number of times every hour. A prison officer would now and then make it harder by tightening the crank with a screw.